GMG-GC: implement free surface

[tjhei]

it currently fails with

An error occurred in line <3827> of file </home/900358141/dealii-experimental/tmp/unpack/deal.II-v9.6.1/source/dofs/dof_handler_policy.cc> in function
    std::vector<dealii::internal::DoFHandlerImplementation::NumberCache> dealii::internal::DoFHandlerImplementation::Policy::ParallelDistributed<dim, spacedim>::distribute_mg_dofs() const [with int dim = 2; int spacedim = 2]
The violated condition was:
    (triangulation->is_multilevel_hierarchy_constructed())
Additional information:
    Multigrid DoFs can only be distributed on a parallel Triangulation if
    the flag construct_multigrid_hierarchy is set in the constructor.

because it assumes we can do a local smoothing GMG for the free surface solve.